Re: [PATCH 1/1] crypto: ccp: use file mode for sev ioctl permissions

[Nathaniel McCallum]

On Sun, Mar 8, 2020 at 5:54 PM David Rientjes <rientjes@xxxxxxxxxx> wrote:
>
> On Fri, 6 Mar 2020, Connor Kuehl wrote:
>
> > Instead of using CAP_SYS_ADMIN which is restricted to the root user,
> > check the file mode for write permissions before executing commands that
> > can affect the platform. This allows for more fine-grained access
> > control to the SEV ioctl interface. This would allow a SEV-only user
> > or group the ability to administer the platform without requiring them
> > to be root or granting them overly powerful permissions.
> >
> > For example:
> >
> > chown root:root /dev/sev
> > chmod 600 /dev/sev
>
> Hi Connor,
>
> I'm curious why do you need to do the two above commands?  It implies that
> /dev/sev is either not owned by root or that it is not already restricted
> to only being owner read and writable.
>
> Or perhaps these two commands were included only for clarity to explain
> what the defaults should be?

Correct. Those are just exemplary. They represent the existing permissions.

> > setfacl -m g:sev:r /dev/sev
> > setfacl -m g:sev-admin:rw /dev/sev
> >
> > In this instance, members of the "sev-admin" group have the ability to
> > perform all ioctl calls (including the ones that modify platform state).
> > Members of the "sev" group only have access to the ioctls that do not
> > modify the platform state.
> >
> > This also makes opening "/dev/sev" more consistent with how file
> > descriptors are usually handled. By only checking for CAP_SYS_ADMIN,
> > the file descriptor could be opened read-only but could still execute
> > ioctls that modify the platform state. This patch enforces that the file
> > descriptor is opened with write privileges if it is going to be used to
> > modify the platform state.
> >
> > This flexibility is completely opt-in, and if it is not desirable by
> > the administrator then they do not need to give anyone else access to
> > /dev/sev.
> >
> > Signed-off-by: Connor Kuehl <ckuehl@xxxxxxxxxx>
> > ---
> >  drivers/crypto/ccp/sev-dev.c | 33 +++++++++++++++++----------------
> >  1 file changed, 17 insertions(+), 16 deletions(-)
> >
> > diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
> > index e467860f797d..416b80938a3e 100644
> > --- a/drivers/crypto/ccp/sev-dev.c
> > +++ b/drivers/crypto/ccp/sev-dev.c
> > @@ -283,11 +283,11 @@ static int sev_get_platform_state(int *state, int *error)
> >       return rc;
> >  }
> >
> > -static int sev_ioctl_do_reset(struct sev_issue_cmd *argp)
> > +static int sev_ioctl_do_reset(struct sev_issue_cmd *argp, bool writable)
> >  {
> >       int state, rc;
> >
> > -     if (!capable(CAP_SYS_ADMIN))
> > +     if (!writable)
> >               return -EPERM;
> >
> >       /*
> > @@ -331,12 +331,12 @@ static int sev_ioctl_do_platform_status(struct sev_issue_cmd *argp)
> >       return ret;
> >  }
> >
> > -static int sev_ioctl_do_pek_pdh_gen(int cmd, struct sev_issue_cmd *argp)
> > +static int sev_ioctl_do_pek_pdh_gen(int cmd, struct sev_issue_cmd *argp, bool writable)
> >  {
> >       struct sev_device *sev = psp_master->sev_data;
> >       int rc;
> >
> > -     if (!capable(CAP_SYS_ADMIN))
> > +     if (!writable)
> >               return -EPERM;
> >
> >       if (sev->state == SEV_STATE_UNINIT) {
> > @@ -348,7 +348,7 @@ static int sev_ioctl_do_pek_pdh_gen(int cmd, struct sev_issue_cmd *argp)
> >       return __sev_do_cmd_locked(cmd, NULL, &argp->error);
> >  }
> >
> > -static int sev_ioctl_do_pek_csr(struct sev_issue_cmd *argp)
> > +static int sev_ioctl_do_pek_csr(struct sev_issue_cmd *argp, bool writable)
> >  {
> >       struct sev_device *sev = psp_master->sev_data;
> >       struct sev_user_data_pek_csr input;
> > @@ -356,7 +356,7 @@ static int sev_ioctl_do_pek_csr(struct sev_issue_cmd *argp)
> >       void *blob = NULL;
> >       int ret;
> >
> > -     if (!capable(CAP_SYS_ADMIN))
> > +     if (!writable)
> >               return -EPERM;
> >
> >       if (copy_from_user(&input, (void __user *)argp->data, sizeof(input)))
> > @@ -539,7 +539,7 @@ static int sev_update_firmware(struct device *dev)
> >       return ret;
> >  }
> >
> > -static int sev_ioctl_do_pek_import(struct sev_issue_cmd *argp)
> > +static int sev_ioctl_do_pek_import(struct sev_issue_cmd *argp, bool writable)
> >  {
> >       struct sev_device *sev = psp_master->sev_data;
> >       struct sev_user_data_pek_cert_import input;
> > @@ -547,7 +547,7 @@ static int sev_ioctl_do_pek_import(struct sev_issue_cmd *argp)
> >       void *pek_blob, *oca_blob;
> >       int ret;
> >
> > -     if (!capable(CAP_SYS_ADMIN))
> > +     if (!writable)
> >               return -EPERM;
> >
> >       if (copy_from_user(&input, (void __user *)argp->data, sizeof(input)))
> > @@ -698,7 +698,7 @@ static int sev_ioctl_do_get_id(struct sev_issue_cmd *argp)
> >       return ret;
> >  }
> >
> > -static int sev_ioctl_do_pdh_export(struct sev_issue_cmd *argp)
> > +static int sev_ioctl_do_pdh_export(struct sev_issue_cmd *argp, bool writable)
> >  {
> >       struct sev_device *sev = psp_master->sev_data;
> >       struct sev_user_data_pdh_cert_export input;
> > @@ -708,7 +708,7 @@ static int sev_ioctl_do_pdh_export(struct sev_issue_cmd *argp)
> >
> >       /* If platform is not in INIT state then transition it to INIT. */
> >       if (sev->state != SEV_STATE_INIT) {
> > -             if (!capable(CAP_SYS_ADMIN))
> > +             if (!writable)
> >                       return -EPERM;
> >
> >               ret = __sev_platform_init_locked(&argp->error);
> > @@ -801,6 +801,7 @@ static long sev_ioctl(struct file *file, unsigned int ioctl, unsigned long arg)
> >       void __user *argp = (void __user *)arg;
> >       struct sev_issue_cmd input;
> >       int ret = -EFAULT;
> > +     bool writable = file->f_mode & FMODE_WRITE;
> >
> >       if (!psp_master || !psp_master->sev_data)
> >               return -ENODEV;
> > @@ -819,25 +820,25 @@ static long sev_ioctl(struct file *file, unsigned int ioctl, unsigned long arg)
> >       switch (input.cmd) {
> >
> >       case SEV_FACTORY_RESET:
> > -             ret = sev_ioctl_do_reset(&input);
> > +             ret = sev_ioctl_do_reset(&input, writable);
> >               break;
> >       case SEV_PLATFORM_STATUS:
> >               ret = sev_ioctl_do_platform_status(&input);
> >               break;
> >       case SEV_PEK_GEN:
> > -             ret = sev_ioctl_do_pek_pdh_gen(SEV_CMD_PEK_GEN, &input);
> > +             ret = sev_ioctl_do_pek_pdh_gen(SEV_CMD_PEK_GEN, &input, writable);
> >               break;
> >       case SEV_PDH_GEN:
> > -             ret = sev_ioctl_do_pek_pdh_gen(SEV_CMD_PDH_GEN, &input);
> > +             ret = sev_ioctl_do_pek_pdh_gen(SEV_CMD_PDH_GEN, &input, writable);
> >               break;
> >       case SEV_PEK_CSR:
> > -             ret = sev_ioctl_do_pek_csr(&input);
> > +             ret = sev_ioctl_do_pek_csr(&input, writable);
> >               break;
> >       case SEV_PEK_CERT_IMPORT:
> > -             ret = sev_ioctl_do_pek_import(&input);
> > +             ret = sev_ioctl_do_pek_import(&input, writable);
> >               break;
> >       case SEV_PDH_CERT_EXPORT:
> > -             ret = sev_ioctl_do_pdh_export(&input);
> > +             ret = sev_ioctl_do_pdh_export(&input, writable);
> >               break;
> >       case SEV_GET_ID:
> >               pr_warn_once("SEV_GET_ID command is deprecated, use SEV_GET_ID2\n");
> > --
> > 2.24.1
> >
> >
>